Today, the most common modality for optical signal amplification is the rare-earth doped fiber amplifier. These devices have good amplification characteristics and a well-understood long-term behavior. Moreover, they can be inserted into a fiber link via fiber splicing, which is a low loss coupling technique.
An alternative amplification modality is the semiconductor optical amplifier (SOA). SOA systems have a number of advantages relative to the common erbium-doped amplifier scheme. SOA""s are typically electrically, rather than optically, pumped. As a result, they are more efficient and avoid the need for ancillary, expensive laser pumps. Moreover, they are usually physically smaller than fiber amplifiers, which require a relatively long length of doped fiber. This quality is especially relevant when amplification is required in larger systems offering higher levels of functionality, such as optical add-drop multiplexers and other types of switching devices. The semiconductor optical amplifier can be as small as the semiconductor chip.
Nonetheless, the principal barrier to the commercial deployment of semiconductor optical amplifiers is the difficulty associated with coupling optical signals in and out of the semiconductor amplifier chip. The coupling issues are analogous to coupling light from a laser transmitter/laser pump into an optical fiber with the additional problems associated with back-reflection suppression, which can convert the amplifier into a laser, resulting in unintended operation.
The present invention concerns a semiconductor optical amplifier system and specifically, the implementation of a semiconductor optical amplifier system on a single substrate or optical bench. As such, the present invention is applicable to the inclusion of a physically-compact amplification system into larger optical systems.
When constructing semiconductor optical amplifiers, it is typically desirable to have some type of feedback mechanism to control the amplification level of the semiconductor optical amplifier. Proposed techniques for diverting a portion of the optical signal from the amplifier chip, however, can be susceptible to polarization shifts in the optical signal. As a result, they can introduce some noise into the feedback scheme.
In general, according to one aspect, the present invention features a semiconductor optical amplifier system. It comprises a hermetic package. In the typical implementation, this hermetic package is a standard 0.75 inchxc3x970.5 inch package, such as a butterfly package. An optical bench is sealed within this package. A first fiber pigtail enters this package via a feed-through to connect to and terminate above the bench. A second optical fiber pigtail enters the package via a second fiber feed-through to connect to and similarly terminate above the bench. A semiconductor amplifier chip is connected to or installed on the bench.
In a preferred embodiment, at least one isolator is included in the hermetic package and specifically on the optical bench for suppressing back-reflections into the fiber pigtail and/or the semiconductor optical amplifier chip. Specifically, in the preferred embodiment, a first isolator suppresses back-reflections into the input or first fiber pigtail and a second isolator suppresses back-reflections into the semiconductor optical amplifier chip.
In the preferred embodiment, additional optical components are provided to facilitate the transmission of optical signals through the system. Specifically, a first collimation lens is installed on the bench between the first isolator and the termination of the first fiber pigtail to improve the collimation of light emitted from the first fiber pigtail. A focusing lens is installed on the bench between the first isolator and the semiconductor optical amplifier chip to couple light from the first isolator into the semiconductor optical amplifier chip. Further, a second collimation lens is installed between the second isolator and the chip to couple light from the chip into the second isolator. Finally, a second focusing lens is installed on the bench for coupling light from the second isolator into the second pigtail.
Although, in the preferred embodiment, discrete optics are used to couple light into and out of the fiber pigtails, in alternative embodiments, fiber lenses may be formed on the fiber endfaces to reduce or eliminate the need for discrete coupling optics between the fiber endfaces and the other components of the system.
According to another embodiment, in a single physical port embodiment, the optical signal is received into the hermetic package by a fiber, focused onto the amplifier chip, reflected to pass back through the chip, and then refocused onto the fiber so that the amplified optical signal exits from the system.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.